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Lime for Olive Tree Health

It has been found that the addition of agricultural lime (calcium carbonate) to many Californian soils, will improve the health, growth and crop of olive trees.

Following are a couple of statements that show the reason for this.

"The optimal soil pH for olives is not known, although olives grow poorly on soils with a pH above 8.5. Some California soils have a pH higher than 7.0 and high levels of calcium. Olive trees grow well in these calcareous soils."
(Californian Olive Production Manual, p.70)

"...This variety has an average oil content of 22-24% when grown in Sfax (Tunisia) whereas in the north this yield drops to 16% and even 13% on the infertile, calcium-poor soils of Cap Bon."
(Olivae No.61, April 1996)

It is interesting to note that wherever olive trees are performing at their very best, the pH of the soil will be found to be somewhere between 7.0 (neutral) and 8.0 (mildly alkaline).

Acid soils have a low pH reading (below 7) and alkaline soils, suitable for olives, have a higher pH reading (above 7). Acidic soils can be improved by the addition of agricultural lime.

The following are a few of the benefits you will gain by adding agricultural lime to acidic soils. Our thanks goes to Olives Australia/DML Aglime for much of the information.

NB. You will have to carry out these measurements prior to planting and installation of an irrigation system.

1. Calcium is an important constituent of cell wall material, adding strength and stability to the plant. Calcium deficiency causes stunted growth in olive trees and also the development of 'soft nose' where the fruit develops a rotten end thus making it completely unfit for sale. 'Softnose' occurs when the fruit begins to change color from green through to black and has occasionally been seen where the variety Sevillano has been unable to get enough calcium.

2. Adequate levels of lime in the soil will reduce Aluminum and Manganese toxicities. The lower the soil pH, the more readily these toxic elements are released into the soil to adversely affect the health of growing plants.

3. Fertilizer efficiency is improved when agricultural lime is added to acid soils. Liming acid soils will increase the uptake of nitrogen, phosphate, potash, sulfur, calcium, magnesium, boron, copper, zinc and molybdenum.

4. Beneficial soil bacteria are generally more prevalent in the sweet alkaline soils rather than acid soils.

5. Agricultural lime improves soil structure and promotes worm activity. The addition of lime to most soils will improve their friability, thus reducing crusting and clodding of heavy soils.

6. The composting of organic matter in the soil is significantly improved, thus contributing to good soil composition and less need for ongoing tillage.

How Much Lime?

Before I plant my olives, how do I know if my soil needs lime? Well firstly, don't guess. Take a soil sample to be tested by a reputable soil laboratory. Because the olive tree is a relatively shallow rooting tree, scrape away the top 4 inches of soil which contains a lot of organic matter and discard it. Then collect about one pound of soil from the 4 - 8 inch. Collect a second sample at around 2 ft deep.

Take the soil samples to the testing laboratory. You will often find that the top sample will have a lower pH (is more acidic) than the bottom sample. This means that by deep ripping to open up the soil to a depth of around 24 - 30 inches before planting your olives, the lower layer with a higher pH will be mixed with the top layer with a lower pH, thus 'leveling' the pH levels.

For olives, if the mixing of the two soil levels still would not bring the pH level to a minimum of 7.0, then the addition of lime into the soil will be very beneficial.

For maximum benefit, the lime should be deep ripped into the soil to a depth of at least 2 ft. Your soil laboratory or fertilizer supplier will be able to suggest how much lime will be required to raise the pH to between 7.0 and 8.0. As a very rough rule-of-thumb, the following rates will apply.

In sandy loam, the approximate amount of lime required to raise the pH of your olive ground to a depth of 2 ft by one pH unit (eg. to go from 6.5 to 7.5) would be 0.6 to 0.8 tons per acre. Remember, this is not just a shallow dressing for shallow rooted vegetables, but a deep ripped application to cover the full depth of the olive roots as they mature over many years. This 0.6 to 0.8 tons per acre would be applied in a 10 ft wide band down the row lines and very thoroughly deep ripped in to a depth of 2 ft and a width of 10 - 13 ft. An unlimed grass strip between the rows would therefore be left unripped. If you choose to rip or lime the entire field, more lime will be needed. Be very careful ripping the entire field if there is a chance of causing excessive erosion problems.

In heavier clay soils, you will probably need at least twice as much lime to raise the pH level by one unit.

Lime is relatively inexpensive and if you can afford to use it, your investment will bring excellent returns over many years.